There are three nodes that can initiate 'GPRS Detach' procedure. Those nodes are the MS, the SGSN, and the HLR.
There are two types of Detach, 'Explicit Detach', and 'Implicit Detach'.
In Explicit Detach, the node explicitly request to make a detach.
In Implicit Detach, The SGSN detaches the MS without notifying it. this may occur because the MS reachability timer expired as example.
GPRS Detach (MS initiated)
- 1.The MS sends 'Detach request' to the SGSN, including the P-TMSI and P-TMSI signature.
- If there is PDPs exist for this MS, the SGSN will delete those PDPs with the GGSN, by sending 'Delete PDP context request. Then, the GGSN will return 'Delete PDP context response'.
- If there was an association established between the SGSN and the MSC/VLR, the SGSN should inform the MSC/VLR by sending 'GPRS detach indication', so that the MSC/VLR removes the association.
- The SGSN returns 'Detach accept' to the MS.
- In case of 3G, the SGSN will remove the PS signalling connection with the MS.
The PDP context is the data session between the MS and the GGSN. we will go deeper in it later.
The 'PS signaling connection' is the Iu connection between the SGSN and the RNC, in addition to the RRC connection between the RNC and the MS.
GPRS Detach (SGSN initiated)
- The SGSN sends 'detach request' to the MS, including the detach type, which indicates if the MS is required to make a re-attach to the network or not.
- If there are active PDP contexts exist , The SGSN will delete them with the GGSN.
- If an association with the MSC/VLR exist, the SGSN will send 'GPRS detach indication' to the MSC/VLR to delete it.
- The MS will reply with 'detach accept'.
- In case of 3G, the SGSN will remove the PS signalling connection between it and the MS.
GPRS Detach (HLR initiated)
- The HLR will send 'Cancel Location' message, including the IMSI of the MS, requesting the SGSN to detach it.
- The SGSN will initiate the 'detach request' as exist in previous section, and it will send 'Cancel Location Acknowledge' to the HLR.